1. Field of the Invention
The present invention relates generally to a SAL seeker, and more specifically to an engineered diffuser utilized in conjunction with a strap down SAL seeker or a velocity pursuit SAL seeker.
2. Description of the Related Art
Semi-active laser (“SAL”) seekers are well known in the art, and a representative example is described in U.S. Pat. No. 6,262,800, entitled “Dual Mode Semi-Active Laser/Laser Radar Seeker”, by Lewis Minor, and assigned to Lockheed Martin Corporation, the assignee of the present application. The '800 patent describes a marking and guidance system for use on military aircraft to support ground operations. With a SAL seeker system, a narrow laser beam is produced and transmitted toward a target from a laser designator aircraft or from a forward observer located on the ground. The operator of the laser designator directs the laser illumination towards a selected target, thereby designating the target.
The laser source is located remote from the target energy transmitter. The SAL seeker, which is disposed on the missile or other weapon, detects the laser illumination reflected from the target. The SAL seeker system includes processing equipment for generating guidance commands to the missile derived from the sensed laser illumination as it is reflected from the target to guide the missile or weapon to the target. Pilots or other users utilize equipment similar to SAL seeker system to identify a target.
SAL seeker systems typically include a detector sensitive to the laser wavelength and a two axis gimbal system that allows the SAL seeker to have a wider field of regard (“FOR”). It should be noted that a sensor has a field of view (“FOV”), but gimbals provide a non strap down seeker with a wide FOR. For a strap down seeker, FOR=FOV. SAL seekers having a gimbal system are well known in the art.
In order to reduce the cost and simplify the complexity of SAL seekers having gimbals, the United States military wants to develop a strap down SAL seeker that eliminates the gimbal system. The elimination of the gimbal system advantageously reduces the complexity and cost of the SAL seeker, but it also disadvantageously reduces the FOR. New strap down SAL seekers, without a gimbal system, are being proposed for many missile applications, because they are relatively small and inexpensive. SAL seekers, however, require a relatively large field of regard, relatively fast optics, anti-scintillation capability and small physical size. The current optical designs that attempt to meet these conflicting requirements, result in a blur spot that is extremely non-uniform. In addition to strap down SAL seekers, there is also a “birdie” or velocity pursuit SAL seeker. This type of seeker has passive (non driven) gimbals that allow the seeker to align to its velocity vector the way a badminton shuttlecock or birdie aligns to its velocity vector. This type of seeker is currently used in laser guided bombs provided by various vendors. Because this type of seeker has passive gimbals, it is not considered to be a strap down seeker, but it has similar requirements and issues as a strap down seeker.
In addition, atmospheric scintillation and uniformity problems further aggravate the performance of SAL seekers. Air turbulence near the ground is a primary source of atmospheric scintillation, and the air turbulence makes it appear to the SAL seeker that the laser spot is jumping or moving randomly. The spot uniformity problem further compounds the situation, because the laser spot typically does not have a uniform energy distribution. A non-uniform distribution of laser energy means that the laser spot does not appear as a well-defined spot of illumination to the detector, and the non-uniformity complicates image processing of the laser spot.
Previous solutions to the scintillation problem have been expensive (use of a fiber optic faceplate) or have produced poor spot uniformity (use of a spherical distortion lens). Moreover, the previous solutions to reduce the scintillation have actually aggravated the uniformity problem. For example, the use of carefully designed aberrations in the optics to reduce scintillation produce significant spot non-uniformity and a non-linear response in a strap down seeker.
Accordingly, there is a need for a SAL seeker that has a relatively wide FOV and overcomes the atmospheric scintillation and spot uniformity problems associated with SAL Seekers, without resorting to the relatively expensive and less satisfactory solutions of the prior art.